sea spider

The Columbia Encyclopedia, 6th ed.

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sea spider, common name for members of the class Pycnogonida, long-legged, rather spiderlike organisms of the subphylum Chelicerata, widely distributed in marine waters. Most are tiny, from 1 to 9 mm (0.04–0.36 in.), and live in littoral regions, crawling about over the surface of sessile animal colonies or seaweeds. Some live on or in clams. There are deep-sea forms, some becoming quite large; Colossendeis colossea has a leg span of nearly 2 ft (91 cm). Their unusual body form makes their relationships to other arthropods obscure. Nearly all of the body is composed of the anterior region (prosoma); a tiny tubular posterior region (opisthosoma) projects behind. A large proboscis is used to suck in food. At the base of the proboscis is a pair of modified appendages (chelicera) used to pick off bits of food and hold them in front of the mouth. The next appendages are a pair of leglike pedipalps, followed by a pair of specialized legs used by the male to carry eggs until they hatch. Four pairs of walking legs follow, but sometimes additional pairs are found. Members of this class are relatively common and widely distributed; well over 400 species are known. Sea spiders are classified in the phylum Arthropoda, subphylum Chelicerata, class Pycnogonida.

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Pycnogonida

Pycnogonida (Pantopoda; sea spiders; phylum Arthropoda, subphylum Chelicerata) A class of marine chelicerates, all of which are small (total body length usually 1–10 mm but up to 6 cm in some species, with a leg span of 75 cm). The narrow body comprises a cephalon and four–six distinct segments, the cephalon bearing a proboscis. The posterior part of the cephalon tapers to form a neck bearing four eyes mounted on a central tubercle on its dorsal surface. There is a pair of chelifores (possibly homologous to arachnid chelicerae), a pair of palps, and two pairs of ovigers (reduced in some species and absent in females of some species). The four–six pairs of eight-segmented walking legs articulate with large processes, pairs of which project from each segment, giving the animal the spider-like appearance to which its common name refers. Some species have eight–ten pairs of legs, possibly as a result of polyploidy. Most pycnogonids are carnivorous and bottomdwellers, although some can swim. There are about 500 species found in all oceans, but more commonly in cold waters, and at all depths.

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